mips64: remove toolchain kludge

-mno-abicalls is an old kludge for some (probably) old issue.
Remove it since it's actually harmful, static busybox doesn't build with
it for a modern-ish toolchain (defaults as of this commit, uClibc
0.9.33.2 + gcc 4.5.4).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>